Cardiac arrest is a significant public health problem cutting across age, race, and gender. A positive impact on cardiac arrest survival has been demonstrated with the substantial reduction in time to defibrillation (the administration of a high energy electrical shock to the heart) provided by the widespread deployment of automated external defibrillators and the use of implantable cardioverter defibrillators (ICDs). Defibrillators have had a major impact on dealing with cardiac arrest in that they are the only reliable treatment for ventricular fibrillation (VF.)
Defibrillators can be implanted or external and can include additional capabilities of cardioversion, bradycardia pacing (brady pacing) and anti-tachycardia pacing (anti-tachy pacing or ATP). A recent development in implantable defibrillators is to design the pulse generator and electrodes to be suitable for subcutaneous implantation, thus avoiding the need for an intracardiac electrode. This method has the advantage of potentially simpler surgical procedures and not requiring fluoroscopic facilities to position an intracardiac electrode. Although less invasive than intra-cardiac electrodes, devices utilizing subcutaneous electrodes must apply relatively higher-amplitude pulses to achieve a comparable therapeutic effect as devices employing intra-cardiac electrodes. This requirement is attributable primarily to the greater distance between the subcutaneous electrodes and the resulting wider, less-focused dispersal of charge into the body, making cardiac cell capture less efficient.
In addition to defibrillation, which is often delivered when the patient is unconscious, modern defibrillators also deliver brady pacing, ATP, and cardioversion therapies. In these therapies, the patient is usually conscious and the discomfort associated with subcutaneous delivery of these therapies (particularly ATP and cardioversion) has prevented their use in subcutaneous devices. A solution is needed to mitigate the discomfort associated with these treatments using subcutaneous implantable pulse generators and electrodes.